a 4000 cu ft room
Son of Ampzilla (nominal 80 WPC)
Sweet, but too small, speakers (not a mainstream brand. Transparent mid & highs, but the 6.5 in woofers are completely swallowed by the room).

I decided that what I need is a pair of small subwoofers (crossed pretty high) to flatten this out. I have picked up a two channel active crossover which has independently adjustable high and low xover frequencies and a NAD 2200PE amp (conservatively rated at 100 WPC) to drive the subs.

I've read the DIY pages and played a little with a couple of the freeware speaker design programs, and I'm on the verge of zeroing in on my drivers and box design. I'm leaning toward Peerless CSX 10" (850146) drivers in sealed, stuffed, 41 liter boxen. (The drivers are not necessarily my first choice, but I need to stay under $75 each.) I tentatively plan to run the mains full spectrum and only cross the subs.

This is a music only system. I don't need to crack the foundation, but I need authority in the bottom, and some help higher up as well. I'm thinking of them more like detached, biamped woofers than true subwoofers.

Before I haul off and spend the money and precious time, am I heading down the right path? Is there a better bargain woofer for my purpose? Is there a better box design? Do I have enough amp for reasonable volumes?

It looks like you have selected a nice driver for the money- it's on my list of woofers to consider in a three way. However, I thought I'd point out that you could save $10 by getting the peerless CC instead of the CSX. It's probably worth the extra money to get the CSX, but if cash is really tight, you have another option.

Modeling that driver in a sealed 41 liter box, I see a Q of 0.769, and F3 of 45 hz, and F10 of 28 hz. Oh, and by the way- I'm using the parameters for this driver as they were preset in WinISD Pro Alpha- slightly different from the listed parameters on PE.

With room gain and a little EQ, this could be a nice addition to your music, but I think that you could get better extension. I won't suggest going ported (I've never really thought it sounded accurate...) but I will say that going to a 70 liter enclosure lowers the Q a bit, and gives about 3 db more output at 20 hz. The difference isn't all that much, maybe it's worth it to have a smaller box.

I say that these drivers look good because of the smooth response and (so I've heard) low distortion. If you have another contender, you might get better extension or sensitivity.

Sounds good. I personally do enjoy the sound of a ported enclosure. To my ears Ported sounds just as good as sealed if properly designed. Thoes seem like some decent woofers for the money, but you will get more surface area and therefore displacement and low end extension from a single 15" woofer. May want to look into that. also, Ported will give you an average of 3dB gain over sealed and a flatter lower extension. My tempest was SAPOSED to be here this weekend. Well it was backordered and wont be here for another 7-10days.... With Valetines day aproching fast i may run out of cash before get the enclosure built (im not doing another half @$$ed job like my current sibwoofer). My girl or my stereo? lol.

What you are planning to do could work quite well. I'm familiar with the NAD, it has nice bass like all NAD power amps, I have also heard the CSX and I wouldn't consider the CC driver, I believe it needs a very big box and is an older design, whereas the CSX has a shorting ring low distortion motor. It also looks quite similar to the much more expensive Focal Audiom drivers, not bad for a budget driver.

What you will get from bi-amping like you are planning is something you might not have anticipated - more headroom and clarity for your mains. If you cross fairly high at say 200 Hz then this reduces the excursion in your mids. The amp driving your mains also has an easier ride and is much less likely to clip, further increasing clean headroom.

What this might not achieve is fully satisfy your desire for bass. I would personally go for a vented alignment. This driver doesn't have the VD to do the job sealed. You typically get up 5 or 6 db of extra output from a vented design, not 3db, partly due to the vented box being bigger. Vented boxes have lower distortion, better linearity and more output, as well as more extension.

What I suggest is start with a vented box, don't work too hard on making it pretty. See if you are happy. If you want more bass, then make a smaller sealed box. Say you might get an F3 @ 60 Hz rolling off at 12db/octave below that. You might then add a 15" sub to fill in from 20 - 60 Hz. If you have a 2nd order lowpass set to 60 Hz they should integrate nicely.

Here are some SPL charts. 2 Peerless 850136 drives. not 850146. sorry i was too lazy to type in a bunch of T/S. SPL@1w*1m
1st. Optimal Enclosures Calulated by WinISD

2nd Both enclosures 29.7l

3rd Both enclosures 47.8l

I said Average, not peak dB gain. Over all you cant extract more energy form the back of the cone than it is produceing. 3dB doubles sound energy. you get a greater gain near port tuning because you are reaching optimal port velocity.

I've noticed the drastically superior plots of the ported enclosure, but I'm shying away from that design for a few reasons:

1) In a tradeoff, I'll take transient response over LF extension.

2) Did I mention that these things have to be as inconspicuous as possible? SAF is a critical metric.

3) This is my first DIY speaker, and I'm not confident that I can get critical factors right. I'm led to believe that there is less to screw up with a sealed enclosure.

On the other hand, I'm beginning to question how much LF I'm willing to trade away, and the issue of distortion is a troubling one. As I mentioned, these will have to cross pretty high (200 hz is a good rough guess) and audible distortion in the upper range here would be very disappointing. Part of the reason for my driver choice is its advertised ability to extend above 'subwoofer' territory.

I chose the enclosure size assuming a 30% effective boost from stuffing the boxes, so I'd end up with something like the 48 liter classic Acoustic Suspension. It's not etched in stone or anything, just the minimum I thought I could get away with.

I guess my question now is... still assuming a small sealed enclosure, how does the box volume affect distortion in the range of 60 - 300 Hz? Is it a critical factor, or do I need to rethink the whole thing, or what?

Ddeele, you (like most of us when we started) are selling yourself short!

Speaker boxes are a very inexact science for all but the most exacting persons and designs. If you miss a half-inch down one side of a ported box you might not even notice by listening to it, compared to one done precisely. Ports are easy to do also. If you can make a sealed box you can make a ported box... it's only another hole or two!

The main thing I wanted to say though is that I hope you plan to deal with localization by putting a sub. right with the main speaker... you will definately be able to tell where 200-300Hz is coming from, best not to make it the opposite side of the room!

Ddeele, am I right in thinking that you are concerned about distortion from excessive compression of the air inside the sealed boxes??
I'm sure some-one with a good mathematical understanding of this issue could give a definitive answer.
My emperical understanding is that this form of distortion would only become an issue in very small enclosure and/or ones were very high SPL's at low frequencies were reproduced (i.e commercial systems).
Does someone out there know the maths?
You are very correct in stateing that sealed boxes are more idiot-proof. But having said that, a lot of idiots (myself included) have made good sounding ported boxes.
If you do go for a sealed design, err on the side of making the box smaller. The "ringing" off an under-damped sealed box will be much more intrusive than any other distortion you might encounter!